1. Field of the Invention
The present invention relates to electronic circuits and signals, and, more particularly, to flux locked loops for providing feedback signals, including feedback signals to direct current superconducting quantum interference devices (DC SQUIDs).
2. Description of the Prior Art
Flux locked loops are typically used in the read-out electronics of magnetic measurement systems employing DC SQUIDs. DC SQUIDs are small, cryogenically-cooled magnetic sensors that comprise a ring of superconducting material interrupted by two Josephson junctions. DC SQUIDs are designed to detect changes in magnetic flux and can be used as flux-to-voltage converters. The standard read-out method for DC SQUID measurements is to inject a magnetic field modulation signal into the DC SQUID and then, using a flux locked loop (FLL) circuit, sense changes in the modulating signal due to external magnetic fields. The FLL provides negative feedback to the DC SQUID in order to maintain linear operation and a stable operating point at the DC SQUID. This negative feedback precisely counteracts the externally applied magnetic field, provided the slew rate and dynamic range of the DC SQUID and FLL are not exceeded. Measurements of the external magnetic flux can be made by measuring this feedback signal which is an identical image of the external magnetic flux signal detected by the pick-up coil of the DC SQUID sensor within the tracking bandwidth of the FLL.
DC SQUID sensor systems for non-destructive testing/evaluation or for biomagnetic measurements are not yet practical for use in a field setting (i.e., environments containing high levels of magnetic interference). The art has been limited to a flux modulation frequency of approximately 500 KHz with a maximum tracking loop bandwidth of 250 KHz. These limitations result in the FLL losing lock when exposed to large amplitude or high slew rate external stray magnetic fields from 50/60 Hz AC power lines, AM broadcast transmitters, small changes in the Earth""s magnetic field, and other sources present in unshielded environments. When the FLL loses lock, any measurement in progress is invalidated.
The FLL of the present invention includes essential enabling technology which makes the operation of DC SQUIDs practical in unshielded environments by alleviating the effects of high levels of magnetic interference on DC SQUID measurements, and improving FLL performance by a factor of at least ten. This is accomplished in part by implementing the FLL with analog and radio-frequency (RF) components. The use of RF techniques is based on an RF analysis of the DC SQUID transfer function. Detailed RF analysis of the signal output from a DC SQUID coupled to a traditional flux locked loop using square wave modulation reveals the signal output to be in the form of double sideband suppressed carrier (DSB-SC) modulation products centered at the modulation frequency and occupying twice the bandwidth of the baseband signal. The use of RF techniques results in a flux modulation frequency of at least 33 MHz and a maximum tracking loop bandwidth of at least 5 MHz. The FLL is thus able to track, without unlocking, undesired high slew rate magnetic interference, thereby eliminating the need for expensive and restrictive magnetic shielding for the DC SQUID.
These and other important aspects of the present invention are more fully described in the section entitled Detailed Description, below.